1. Field of the Invention
This invention relates to compressors used in heavy vehicle braking systems. More particularly, the application is directed to an oil-less/oil free air compressor.
2. Discussion of the Art
Air compressors are used in brake systems to provide and maintain air under pressure to operate the vehicle brakes and any auxiliary air systems. The compressor is engine driven and typically is a two cylinder, single stage, reciprocating compressor. A connecting rod extends from the engine driven crankshaft and is operatively connected to a piston that reciprocates in an associated bore to compress the air in the bore and provide pressurized air to the brake system/auxiliary air system.
The vehicle engine provides a continuous supply of oil to the compressor. The oil is routed from the engine to an oil inlet of the compressor to maintain lubrication of connecting rod and crankshaft bearings, piston rings and other dynamic components. The pistons typically include a plurality of piston rings to seal with the bore wall. For example, commercial arrangements usually employ five (5) piston rings that, although seal the compression chamber, do not inhibit sufficiently oil thrown from the crankshaft from entering into and contaminating the air brake system.
A parameter PV is usually associated with heat flux imposed by the rubbing surfaces of compressors. A PV factor identifies the severity of wear associated with the rubbing components. In connection with piston rings, a PV factor is recognized as a product of average per cycle gas pressure (represented, e.g., in pounds per suare inch (psi)) multiplied by the average ring velocity in the reciprocating motion (expressed, e.g., in feet per minute (fpm)). By way of example only, a typical air brake compressor has a PV factor in the range of 32,000 psi-fpm. The PV factor is one indicator of the wear of the piston rings. The less the PV factor, the less severe is wear and the operation is improved.
It is known that reducing the length of the stroke of the piston would, in turn, reduce the linear speed of the piston and thus have an impact on the PV factor. However, this would necessitate larger pistons or more pistons to compensate for a reduced amount of compressed air. The dimensions of the system that accommodate the air compressor do not permit the mere addition of similar pistons or substitution with a larger piston. Thus, a need exists to convert the compressor system into a multi-cylinder system without appreciably expanding the dimensions of the original compressor arrangement.
Piston rings of the oil-less/oil-free compressors are usually constructed from polymeric materials that are subject to degradation at elevated temperatures. Thus, a continued need exists to reduce the heat imposed on the piston rings to maximize the useful life of the ring.